Indication of fluid or liquid level is a critical variable in process control, storage tank monitoring, and mechanical systems where liquids are contained. Level monitoring systems for liquids typically comprise a transducer to convert liquid level to an appropriate level indicating variable or signal, a transmission medium for conveying the variable or signal, a receiver to accept and process the variable or signal, and a display for visual indication. Liquid level indication is of particular importance in the marine industry. For example, when boats travel in the open sea, an accurate indication of fuel quantity is an essential component to successful navigation.
In addition to accurate indication of fuel quantity, it is also valuable to have an alarm mechanism that warns of dangerously low or high fluid levels. In the marine industry, warning of low fuel level is important so that marine craft operating personnel can timely plan a fueling station visit. Because fuel dispensing units at most marine fueling stations lack the same automatic shutoff capability that automobile fuel dispensers possess, the risk of fuel spillage from tank overfill is quite high. Fuel spillage contaminates the water and surrounding environment and presents a fire safety danger. Thus, alarm warning of high fuel level is important so that fuel pumping can be stopped to avoid fuel tank overfill.
Fuel level alarm systems are available but a common problem with traditional marine fluid level alarm systems is that source power is switch controlled by the ignition key. Other traditional alarm systems obtain power directly from ship instrumentation. These systems typically employ a timing circuit to shut off power after a predetermined time in order to prevent battery drain. By energizing the alarm system through the instrumentation, however, all on-board instrumentation will be energized since instrumentation is typically daisy-chain wired. Because marine craft fueling typically occurs while at dock, maintenance personnel are at often at work on marine craft systems while fueling occurs. With traditional alarm systems that require energizing the instrumentation to operate, such maintenance personnel are at risk to electrical shock since components may be unexpectedly energized.
Accordingly, what is needed is an alarm system that properly alarms for low-level but can also be safely used for high-level monitoring and control.